Advanced Programming in the UNIX® Environment

ptg10805159

Section 10.14 sigactionFunction 349

The process blocksSIGQUIT,saving its current signal mask (to restorelater), and then

goes to sleep for 5 seconds. Any occurrence of the quit signal during this period is

blocked and won’t be delivered until the signal is unblocked. At the end of the

5-second sleep, we check whether the signal is pending and unblock the signal.

Note that we saved the old mask when we blocked the signal. To unblock the

signal, we did aSIG_SETMASKof the old mask. Alternatively, we couldSIG_UNBLOCK

only the signal that we had blocked. Be aware, however, if we write a function that can

be called by others and if we need to block a signal in our function, we can’t use

SIG_UNBLOCKto unblock the signal. In this case, we have to useSIG_SETMASKand

restorethe signal mask to its prior value, because it’s possible that the caller had

specifically blocked this signal beforecalling our function. We’ll see an example of this

in thesystemfunction in Section 10.18.

If we generate the quit signal during this sleep period, the signal is now pending

and unblocked, so it is delivered beforesigprocmaskreturns. We’ll see this occur

because theprintfin the signal handler is output beforetheprintfthat follows the

call tosigprocmask.

The process then goes to sleep for another 5 seconds. If we generate the quit signal

during this sleep period, the signal should terminate the process, since we reset the

handling of the signal to its default when we caught it. In the following output, the

terminal printsˆ\when we input Control-backslash, the terminal quit character:

$./a.out

ˆ\ generate signal once (before5seconds areup)

SIGQUIT pending after return fromsleep

caught SIGQUIT in signal handler

SIGQUIT unblocked after return fromsigprocmask

ˆ\Quit(coredump) generate signal again

$./a.out

ˆ\ˆ\ˆ\ˆ\ˆ\ˆ\ˆ\ˆ\ˆ\ˆ\ generate signal 10 times (before5seconds areup)

SIGQUIT pending

caught SIGQUIT signal is generated only once

SIGQUIT unblocked

ˆ\Quit(coredump) generate signal again

The messageQuit(coredump)is printed by the shell when it sees that its child

terminated abnormally.Note that when we run the program the second time, we

generate the quit signal ten times while the process is asleep, yet the signal is delivered

only once to the process when it’s unblocked. This demonstrates that signals arenot

queued on this system.

10.14 sigactionFunction

Thesigactionfunction allows us to examine or modify (or both) the action associated

with a particular signal. This function supersedes thesignalfunction from earlier

releases of the UNIX System. Indeed, at the end of this section, we show an

implementation ofsignalusingsigaction.